This invention relates to a charged particle generating device used in an ion implanter, a quadrupole mass spectrometer, etc. and in particular to a charged particle generating device, which can be easily mounted and which has a simple construction or a high sensitivity, as well as a focusing lens suitable for focusing or accelerating or decelerating a charged particle beam therein.
An ion implanter, a quadrupole mass spectrometer, a secondary ion mass spectrometer, etc., can be cited as the charged particle generating apparatus. In these apparatuses a focusing lens is used for focusing or accelerating or decelerating charged particles such as ions, which are to be implanted in a semiconductor wafer, etc., ions generated by an object, which is to be analyzed, etc. As the focusing lens an electrostatic lens, in which parallel plane plate electrodes are combined to form a plurality of stages, as described in JP-A-59-40448 and JP-A-62-217549, is used widely. FIG. 15 is a cross-sectional view illustrating schematically a prior art electrostatic lens. Five electrodes 40, 41, 42, 43 and 44, which are parallel plane plates, each of which has a hole at the central portion, through which a beam passes, are combined. The electrodes 40, 42 and 44 are earthed and the electrodes 41 and 43 are connected with a power supply 33 so that a positive or negative high voltage is applied thereto. In this way a two-stage unipotential lens is formed. Further, in the quadrupole mass spectrometer described in JP-A-59-230246, there is disposed a decelerating zoom lens between an ion source and an analyzer, in which extracting electrodes are combined to form a plurality of stages, so as to obtain a low energy ion beam of several eV.
The prior art techniques described above have a good focusing property for a high quality beam such as a beam emitted by a point source, a parallel beam, etc., but for a beam having a great angular dispersion or a great energy dispersion emitted by a spread source the focusing property is restricted. In order to reduce spherical aberration produced by differences in positions for the purpose of obtaining a charged particle beam of high quality from a spread source according to the prior art techniques, it is necessary to increase the diameter of the holes, through which the beam passes, to a value several times as great as the diameter of the beam. In addition, in order to reduce chromatic aberration produced by differences in the energy of charged particles, it is necessary to increase the number of electrodes. However, in this case, there are problems that the electrode system becomes large and that high precision mounting is necessary in order to prevent worsening of the property due to decentering at the mounting. Furthermore, there are problems that mounting operability as a charged particle generating device is worsened, that the construction of the device itself is complicated, that the property as an analyzing apparatus is lowered, etc. Similar problems due to the necessary high precision mounting take place also in a radio frequency quadrupole accelerator.